1. Field of the Invention
The present invention relates to microbiological industry, specifically to a method for producing amino acids. More specifically, the present invention concerns a method for producing L-leucine using bacterium belonging to the genus Escherichia wherein the amount of L-valine, L-isoleucine and L-homoleucine produced is less than 1% of that of L-leucine produced.
2. Description of the Related Art
Conventionally L-amino acids have been industrially produced by a method of fermentation utilizing strains of microorganisms obtained from natural sources or mutants of the same especially modified to enhance the L-amino acids productivity.
Various strains belonging to the genus Escherichia used for production of L-leucine by fermentation are known. There are strains resistant to L-leucine and its analogs, such as 4-azaleucine or 5,5,5-trifluoroleucine (U.S. Pat. No. 5,744,331), β-2-thienylalanine and β-hydroxyleucine (U.S. Pat. No. 5,763,231), L-valine, 4-azaleucine, 3-hydroxyleucine and L-leucine (Russian patent RU 2140450); strains requiring lipoic acid for growth (U.S. Pat. No. 6,214,591); strains with increased activities of the enzymes involved in L-leucine biosynthesis, such as ilvE gene product (U.S. Pat. No. 5,120,654); strains with the target enzymes desensitized to the feedback inhibition by produced L-leucine, such as isopropylmalate synthase (European patent EP1067191).
The most known L-leucine producing strains simultaneously produce L-valine and in the small extent L-isoleucine. For example, E. coli strain AJ11478 (U.S. Pat. No. 5,763,231) produces simultaneously 1.9 g/l of L-leucine and 0.09 g/l of L-valine (amount of L-valine is 4.7% of L-leucine amount). L-valine and L-isoleucine produced simultaneously with L-leucine inconvenience the recovery of L-leucine from cultural liquids. Besides, L-valine and L-isoleucine co-production decreases L-leucine production since both amino acids are originated from a common precursor, 2-ketoisovalerate.
Earlier it was shown, the unnatural amino acids, such as norvaline, homoisoleucine and norleucine, could be formed by L-leucine biosynthetic enzymes in Serratia marcescens from α-ketobutyrate, α-keto-β-methylvalerate and α-ketovalerate, respectively (Kisumi M., Sugiura M. and Chibata I., J. Biochem. 1976, 80(2) 333–9).